The present invention relates to a communication system for making communication using videos, sounds, and electronic data, wherein a band range of the transmission line is multiplexed by means of multiplex techniques such as frequency multiplex, time multiplex, and the like.
Normally, in case of an RF (Radio Frequency) signal, data is kept transmitted continuously like TV and radio broadcasting. In the receiver side, a tuner is synchronized with a characteristic frequency to receive the signal. Therefore, if other data than the signal exists in the frequency range, this data is received as normal data and causes noise. For example, in case of TV broadcasting, a ghost appears due to a reflected wave which is received after a delay.
In a technique which has already been established, video and audio data is subjected to A/D conversion, compressed according to MPEG, modulated in a method of QPSK, QAM, or the like, and transmitted on an RF signal. Therefore, the band range necessary for one channel is small. In general, however, a plurality of programs are transmitted within a band range of one channel in order to maintain compatibility with existing systems.
In this case, to receive a particular channel, tuning is set to the frequency of the channel of a program and data is demodulated. Thereafter, only particular data is filtered from a plurality of channels, based on an ID which the data has, and the data is D/A-converted. Thus, a particular program can be received and viewed/listened.
That is, as shown in FIG. 1, a plurality of programs can be multiplexed on one same channel at one same frequency, by adopting time-sharing in which multiplex is made in the time axis direction.
However, it is impossible to know the timing when the data to be received is transmitted, even tuning is set to a target frequency. Therefore, it is necessary to keep continuously receiving data, so that even a signal delayed due to reflection or so is received and causes a factor of deterioration of receiving performance.